The invention relates to a method for manufacturing a shrink fabric to be used for filtering, the shrink fabric comprising a seam assembly, weft threads and warp threads.
The invention further relates to a shrink fabric to be used for filtering, the shrink fabric comprising a seam assembly, weft threads and warp threads.
Shrink fabrics used in drum filters are usually two-layer shrink fabrics, because they offer better filtering efficiency and operational reliability than corresponding single-layer shrink fabrics. Shrink fabrics are made of a fabric woven of warp threads running in the longitudinal direction of the weaving machine and weft threads running in its cross, or lateral, direction, the threads being interwoven in the weaving. The threads in the shrink fabric are arranged so that the wefts run substantially in the longitudinal direction of the shrink fabric, the longitudinal direction of the shrink fabric meaning the direction arranged substantially parallel with the peripheral direction of the drum, and the warps run substantially in the lateral direction of the shrink fabric, the lateral direction of the shrink fabric referring to the direction arranged parallel with the drum axle. It should be noted, that in this context the term thread is used to refer to both mono- and multifilament threads made of a material that usually comprises a polymer, such as polyester, polyvinylidene fluoride, polyphenylene sulfide, polyetherether ketone, polyamide, polypropylene or some other suitable material, the cross-section of the threads being annular, elliptic, rectangular or any other shape known per se. By varying fabric structures and their combinations, a shrink fabric with the desired characteristics for different purposes and circumstances of use can be obtained. For liquid to be efficiently separated from a mixture of liquid and solids, the liquid permeability of the shrink fabric must be balanced with its solids holding characteristics. Other important characteristics affecting the utilization and costs of the shrink fabric, which is often used in most demanding circumstances, are its dimensional stability and durability.
The shrink fabric is arranged onto the drum so that the weft threads run in the peripheral direction of the drum and the warp threads in its axial direction. The fabric ends are joined together by means of a seam assembly made using fixing members, such as a seam thread arranged through seam loops, a zipper or any other fixing member known per se, the fabric being then shrunk onto the drum by means of heat. When the drum filter rotates about its axle, the solids that accumulate onto the surface of the fabric are scraped using a doctor blade arranged adjacent to the shrink fabric, substantially parallel with the drum axle. The seam assembly formed when the ends of the fabric are joined together is slightly thicker than the fabric and therefore the seam touches the doctor blade. This takes place every time the drum revolves and it causes an impact-type load on the seam assembly and the entire shrink fabric; the load thus stressing and wearing the seam and the shrink fabric often considerably reduces the service life of the shrink fabric. Due to the load on the seam, the shrink fabric has to be serviced and replaced more often than the durability of the seam would require. This causes additional costs arising from the servicing and purchase of the shrink fabric and unnecessary down time caused thereby. If the doctor blade is arranged at a distance required by the seam assembly from the shrink fabric surface, the doctor blade does not remove all the solids from the surface, which reduces the efficiency of the filter.
An object of the present invention is to provide a method for manufacturing a shrink fabric, and a shrink fabric where the above problems do not appear.
The method of the invention is characterized in that the seam assembly is arranged substantially diagonally with respect to the lateral direction of the shrink fabric.
The shrink fabric of the invention is characterized in that the seam assembly is arranged substantially diagonally with respect to the lateral direction of the shrink fabric.
An essential idea of the invention is that the seam assembly of the shrink fabric is arranged substantially diagonally with respect to the lateral direction of the fabric. In this context, diagonal means that the direction of the seam deviates substantially from the lateral direction of the fabric. A preferred embodiment is based on the idea that the seam assembly is at an angle of 15 to 75xc2x0 with respect to the lateral direction of the fabric. Further, according to another preferred embodiment, the angle of the seam assembly is substantially 45xc2x0 with respect to the lateral direction of the fabric. The method of the invention is based on the idea that when the shrink fabric is manufactured, the forward edge and the rear edge of the fabric that are to be joined together are at a diagonal angle to the lateral direction of the fabric, the direction of the seam assembly arranged on the drum being substantially diagonal with respect to the axial direction of the drum.
An advantage of the invention is that the forces and loads caused by the doctor blade and acting on the seam assembly and, through the seam, on the entire fabric are substantially reduced because only a very small portion of the seam, i.e. the foremost portion in the direction of rotation of the drum, meats the doctor blade suddenly and abruptly, the contact between the seam and the doctor blade being thereafter substantially constant, without sudden loads. In addition, the seam is longer than a prior art seam and therefore the forces acting on the fabric in the peripheral direction of the drum, i.e. in its rotational direction, cause a lower tension to the seam. On the whole, the service life of the shrink fabric is increased and its operational reliability improved, which reduces the costs of purchase, installation and use of the shrink fabric.